The present invention relates to a rotary wing aircraft, and more particularly to an in-line swash plate assembly for controlling the main rotor blade pitch of a rotary wing aircraft main rotor system.
Control of a helicopter is affected by varying the pitch of the rotor blades individually as the rotor rotates and by varying the pitch of all of the blades together. These are known respectively as cyclic and collective pitch control. Blade pitch control of a rotary wing aircraft main rotor is typically achieved through a swash plate assembly which transfers the motion of non-rotating control members to the rotating members.
The swash plate assembly is typically concentrically mounted about a rotor shaft. The swash plate assembly generally includes two rings connected by a series of bearings with one ring connected to the airframe (stationary), and the other ring connected to the rotor hub (rotating). The rotating ring is connected to the rotor hub through a pivoted link device typically referred to as “scissors”, with the static ring similarly connected to the airframe. The rotating swash plate rotates relative the stationary swash plate. Apart from rotary motion, the stationary and rotating swash plate otherwise move as a unitary component.
Control rods mounted to the main rotor blades are connected to lugs formed on the rotating swash plate and operate to transfer loads between the swash plate and the main rotor blades. Main rotor servos extend between and attach to the stationary swash plate and the aircraft fuselage. Displacement of the servos results in corresponding displacement of the stationary swash plate. Hence, by actuating selected servos, collective and cyclic commands are transferred to the rotor head as vertical and/or tilting displacement of the stationary and rotating swash plates.
Collective control is achieved by translating the swash plate assembly up and down with respect to the rotor shaft and cyclic control is achieved by tilting the swash plate relative to the rotor shaft. The stationary ring is typically mounted about the rotor shaft through a spherical ball joint or uniball that allows for tilt of the swash plate assembly, with the standpipe surrounding the rotor shaft allowing translation of the swash plate assembly. The pitch links connect the rotating ring of the swash plate assembly to the pitch or control arms of the rotor blades. The stationary swash plate assembly of the swash plate assembly is positioned by servos which are actuated in response to the pilot's control signals. Thus, when the pilot inputs a control command, the stationary swash plate assembly to be raised, lowered or tilted and the rotating swash plate assembly of the swash plate assembly follows to a collectively or cyclically position the rotor disc.
Certain overall axial and width requirement are required for the swash plate assembly linkages to operate properly. However, to facilitate transportation within a cargo aircraft and/or provide accommodation within a ship hangar, modern rotorcraft, particularly military rotor craft must offer compactness of the main rotor and associated swash plate assembly. Such compactness may be limited by the interference between the linkages and by the maximum swiveling angles permitted by the construction of the articulatable joints and links.
As swash plate assemblies are subject to a substantial amount of movement, wear is typical and these parts are generally high-maintenance components. In addition, the linkages are particularly vulnerable when used in military aircraft where a ballistic impact may cause failure of the swash plate assembly control system. Typically, the more compact the swash plate assembly, the more complicated the linkage geometry and the greater the number of linkages required to achieve a desired range of motion. Furthermore, the articulatable linkage geometry directly effects the loads applied to the swash plate assembly. The loads applied to the swash plate assembly during normal operation include shear, bending, and torsional loads. Furthermore, the more compact the swash plate assembly and the more complicated the linkage, the longer the load path through the swash plate assembly. Various prior art swash plate assembly designs such as “in-line” and “over-under” designs provide various tradeoffs of compactness, complexity and length of load paths.
Accordingly, it is desirable to provide an uncomplicated, short load path swash plate assembly which provides the desired compactness.